This invention relates to a replaceable sealing means and to a replaceable sealing means system for providing fluid-tight sealing means defining an annularly-shaped, axially-extending channel of a rotary vacuum filter, such as the rotary vacuum filter employed in the manufacture of cellulosic papermaking pulp.
Rotary vacuum filters are known in the prior art. Typically, the filter drum is mounted for rotation about a central axis and has a hollow trunnion disposed at one end. The hollow trunnion is connected at its inner end to the drum end wall and rotatably connected at its outer end to a stationary vacuum discharge pipe. The stationary vacuum pipe is in communication with a source of vacuum to drain the filtrate material from the drum as it rotates about its central axis. A vat insert assembly (stuffing box) extends axially about the hollow trunnion. The assembly comprises a radially-extending vat wall joined to a vat insert member including concentrically-disposed respective inner and outer axially-extending, annularly-shaped vat insert elements.
A means defining an annularly-shaped, axially-extending channel is formed between the respective outer surface of the discharge pipe and the inner wall of the hollow trunnion, and between the respective inner surface of the inner vat insert element and the outer surface of the hollow trunnion. Stationary abrasive packing material fills both of these channel means and provides a direct fluid-tight seal by direct engagement between the packing material and the adjacent surfaces of the vat insert or the pipe and the trunnion, respectively. The stationary packing material bears against, for example, the wall of the rotatable trunnion and subjects such trunnion wall to substantial direct abrasive wear. A mechanical follower engages the packing material and holds same in position within the channel means in direct communication with the surfaces forming such channel means. In the case of the trunnion, this presents a substantial problem. The inner wall of the trunnion is typically lined with an abrasion- and chemical-resistant material, such as stainless steel, for withstanding the abrasion of the packing seal material and the chemical action of the pulp filtrate liquid which passes through the trunnion. When this lining no longer protects the inner wall of the hollow trunnion, it will be subject to rapid wear.
When the trunnion wears to the point that liquid cannot be effectively contained by the use of the abrasive packing sealing material and/or the drum filter cannot maintain sufficient vacuum to effect proper operation, the only means for correcting these problems is by replacing the entire trunnion itself. This results in a significant expense to the user because of the high replacement cost of a new trunnion, and because the trunnion installation process is time consuming and labor intensive. For instance, in addition to the cost of purchasing the packing material and mechanical follower, which amounts to about $2,500, the cost of a new trunnion is about $15,000, and the maintenance costs for replacing the trunnion are about $20,000 to $30,000.
The packing material itself also wears out because of the above-described abrasive interaction. This also causes substantial leaking, for instance, of the pulp filtrate liquid from the point of rotatable communication with the trunnion. A significant loss of vacuum in the drum filter may also result.
The most substantial cost to the manufacturer, however, results from the losses incurred due to unscheduled production downtime. More specifically, in order to replace a trunnion, the drum filter must be shut down for a considerable period of time, i.e., an average of 24 to 48 hours, while the faulty filter drum is repaired. Using a conservative estimate of $5,000 for each hour of lost production downtime, the average 24 to 48 hour downtime period for replacing the trunnion could result in a production loss to the manufacturer of almost a quarter of a million dollars.
Thus, there is a clear commercial need for a water-tight sealing system for use in rotary drum filters which does not subject the rotatable trunnion to direct excessive abrasive wear, and which is easily replaceable without removal of the trunnion, so that production downtime can be minimized and maintenance costs significantly reduced.